Improve backward compatability #249

Job	Run time
Bitrot check	1m 38s
Book tests	1m 14s
Build target wasm32-unknown-unknown	1m 37s
Build target wasm32-wasi	1m 27s
Intra-doc links	1m 17s
Rustfmt	14s
Test on ubuntu-latest	7m 7s
Test on i686-unknown-linux-gnu	12m 58s
Test on windows-latest	10m 11s
Test on i686-unknown-linux-gnu with beta features	12m 35s
Test on macOS-latest	7m 21s
Test on i686-unknown-linux-gnu with nightly features	14m 51s
Test on ubuntu-latest with beta features	7m 7s
Test on windows-latest with beta features	10m 12s
Test on macOS-latest with beta features	12m 26s
Test on ubuntu-latest with nightly features	8m 2s
Test on windows-latest with nightly features	11m 30s
Test on macOS-latest with nightly features	14m 24s
	2h 16m 11s

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